Vitreous enameling of ferrous metal



June 6, 1950 A. E. CHESTER vrmsous zmumuuc; 0F mous mm,

2 Sheets-Sheet 1 Filed Aug. 9, 1945 June 6, 1950 A. E. CHESTER 2,510,071

VITRBOUS ENAIELING OF FERROUS IETAL Filed Aug. 9, 1945 2 Sheets-Shoot 2on! m: rm:- 9 J 0. c, ornrenoroe MMN Patented June 6, 1950 VITREOUSENAMELING OF FERROUS METAL Allan E. Chester, Highland Park, 11].,assignor to Poor & Company, Chicago, lit, a corporation of DelawareApplication August 9, 1945, Serial No. 609,759

1 12 Claims.

This invention relates to vitreous enameling, more particularly to newand improved base stock for vitreous enamels, and to a. new and improvedmethod of controlling the adherence oi vitreous enamels to ferrousmetals.

As is well known, one of the principal problems in the art of vitreousenameling is to secure proper adherence between the vitreous enamelcoating and the base stock, to which the enamel is applied, without theuse of a "ground or grip coat containing cobalt, maganese and usuallynickel. It has long been recognized that it would be desirable to employas the enamel base stock, ordinary carbon steels containing the usualpercentage of carbon (e. g., 0.10% to 0.20%), but such steels arecharacterized by rather poor warping properties as compared with thespecial enamel irons or enamel steels when exposed to heat during theburning operation. As a result, the most satisfactory vitreous enamelbase stocks have been specially prepared very low carbon steels(containing, say, 0.8. .70 to 0.02% carbon) which are more expensivethan many of the ordinary steels.

One oi the objects of this invention is to provide a new and improvedmethod of controlling the adherence of vitreous enamels to ferrous metalbase stocks.

Another object of the invention is to provide a new and improved methodof controlling the oxidation and reduction reactions which occur duringthe firing 01' a vitreous enamel.

A further object of the invention is to provide a new and improved typeof material which is specially adapted to receive a coating of avitreous enamel.

Still a further object of the invention is to provide a new and improvedcomposition of matter containing thereon a coating or layer or anoxidizing agent sealed by another coating of a metal which is reducingin relation to iron upon thermal excitation.

An additional object is to prepare new and improved enamel base stocksfrom ordinary steels.

Other objects and advantages of the invention will appear from thefollowing description in conjunction with the accompanying drawings, inwhich:

Fig. 1 represents in perspective a sheet of ferrous metal base stockwith an oxidizing layer formed on opposite sides thereof in accordancewith the invention;

Fig. 2 illustrates the i'errous metal base stock with an oxidizing layerand a reducing metal 2 layer on opposite sides thereof, as provided inaccordance with the invention;

Figs. 3 and 4 illustrate a. ferrous metal base stock corresponding,respectively, to that shown in Figs. 1 and 2, but treated on one sideonly; and

Fig. 5 illustrates diagrammatically one type of apparatus which can beemployed in the treatment of the base stock material.

Generally stated, the invention is based upon the discovery that theadherence of vitreous enamels to a ferrous metal base stock can becontrolled by subjecting the base stock to a controlled treatment toform an oxidizing layer thereon and thereafter firing the vitreousenamel thereon in the presence of a metal which is reducing with respectto ferrous iron under the firing conditions.

This can be accomplished in several ways, but is preferably efiected inaccordance with the invention by applying an oxidizing layer to aferrous metal base as a thin coating, and sealing said coating or layerexteriorly by another coating or layer of a metal which is reducing inrelation to ferrous iron upon thermal excitation. The oxidizing agentused to form the oxidizing layer may be any oxidizing agent capable 01'ex!- dizing the reducing metal under enamel firing conditions. In otherwords, the oxidizing layer is oxidizing with respect to ferrous iron.The reducing metal can be any metal 01' alloy of metals below iron(Fe++) in the electrochemical series (as listed, for example, in theChemical Rubber Handbook, 19th edition, page 850) capable of forming acoating which is permanent for ordinary handling and usage and whichdoes not flash oil or sublime at temperatures up to vitreous enamelfiring temperatures.

The vitreous enamel firing temperatures will vary, depending upon thecomposition of the vitreous enamel. For example, a high firing vitreousenamel may be fired at temperatures as high as 1650 degrees Fahrenheitwhile a low firing vitreous enamel may be fired at temperatures around1250 degrees Fahrenheit, or below. The invention especially contemplatesa vitreous enameling method which makes it possible to employ low firingvitreous enameling compositions which can be fired at much lowertemperatures than heretofore considered feasible. The minimum firingtemperature will naturally vary, depending upon the type of vitreousenameling composition, but vitreous enameling compositions firing attemperatures as low as 800 degrees Fahrenheit, or lower. arecontemplated in ac-- cordance with the invention.

Apparently, there is a quantitative relation between the amounts of theoxidizing layer, the reducing metal, and the firing temperatures. Thus,a higher oxidation rate is obtained at the higher firing temperaturesand a smaller amount of oxidizing layer is required to obtain optimumadherence. On the other hand. at the lower firing temperatures. a largeramount of the oxi izing layer is required to obtain optimum adherence.The invention is not l mited to the use of any particular type ofoxidizing layer, but good results have been obtained in practice byapplying to the ferrous base stock a coating of nescently formedcolloidal sulphur, such as is precipitated by adding an aqueous solutionof sodium polysulphide, potassium polysulphide or other polysulphides toan aqueous sulphuric acid solution containing from 6% to of H9504 byweight. However, other oxidizing agents or materials, either elementalor in the form of compounds, organic or inorganic. which are ca ble offorming a deposit. layer or coating oxidizin with respect to ferrousiron on the ferrous base metal material. can be em loyed in accordancewith the invention. Examples of such substances are sel nium. manganesedioxide and ox dizing chlorides. chlorateanitrates and for or anicoxidiv ng agents. such as for exam le. quinones.

The invention will be further illustrated but is not limited by thefollowing example:

Example A cold rolled sheet metal base stock cons stin of S. A. E. 1020carbon steel was placed in an acid ickling bath for a period of three totwelve minutes.

The acid pickling bath was an aoueous bath containing 8% by weight ofsulphuric acid. 1% of lactic acid. and 1 ounce per allon of sodium acidfluoride (Nat-IF) heated at a temperature of 140 de re s to 150 degre sFahrenheit.

The pickled sheet was then rinsed and ou into a sodium polysulphidesolution. The sodium polysulphide solution was made by dissolvin 12.5pounds of sodium sulphide (NmS) in 4.5 gallons of warm water togetherwith 4.25 pounds of elemental sulphur, and refluxing for two and onehalfhours at 228 degrees Fahrenheit. This bath was then slowed to cool andthe pickled metal sheet was placed therein at room temperature (75degrees Fahrenheit).

The metal sheet was then connected to one pole of alternating currenttransformer having a potential of 6 volts (R. M. S.) and the other polewas connected to a graphite electrode. The sheet was subjected to acurrent having a density of 80 amperes per square foot for one minute,thereby forming a sheet having an oxidizing layer thereon which wasoxidizing in relation to ferrovs iron, as illustrated in Fig. 1.Alternatively, both legs of the transformer can be connected to graphiteelectrodes and the sheet suspended between the electrodes in thepoiysulphide solution.

If desired, a wetting agent can be added to the polysulphide bath toincrease the rate of drainage after the sheet has been taken from thebath. Any suitable wetting agent can be employed, such as, for example,n by weight of lauryl sulphate, or a correspondingly eii'ective amountof some other high alcohol sulphate or long chain sulphate or sulphonatecontaining 8 to 18 carbon atoms.

The metal sheet turned a brownish black 4 color during the deposition ofthe sulphur. and the latter was probably entrained in a light hydratedand spongy iron oxide layer on the surface of the metal. When the sheetwas taken out of the bath, it was spray-rinsed with water. thenimmersion-rinsed, and thereafter was made a cathode in a nickel-cobaltplating bath havins the following composition. It is my belief that ironsalts. mainly ferrous sulphate, are entrained in the pickle pits formedby the attack of the acid on the ferrous metal surface and that they arenot completely removed by the rinsing operations, that these iron saltsare hydrolyzed upon immersion in the sulphide bath and that colloidalsulphur is precipitated and a certain amount of iron sulphide is formed.

the remaining being water.

This bath contained a metal ratio of 69.8 grams of nickel and 1.15 gramsof cobalt per liter of solution. The pH of the bath was approximately3.9. The boric acid was employed as a buil'er.

The sulphur coated metal sheet was plated with an electrical hook-up ofthe type shown in Fig. 5 in which the plating bath was contained in areceptacle provided with one or more electrodes, generally illustratedat 2, and the surphur coated metal sheet formed the cathode, asrepresented by the numeral 3.

The anode 2 was connected by suitable eonductors to the secondary 4 of apower transformer having a core 5 and a primary 6. The primary 8 wasconnected to a regulating autotransformer, generally illustrated at I,which in turn was connected to a suitable source of electrical energy,such as a 220 volt or 440 volt, cycle, alternating current. Directcurrent was applied by means of a low voltage current generator,generally shown at B, one side of which passed to the workpiece 3through a rheostat 9, and the other side of which was connected to thesecondary l, in the manner shown. The ratio of alternating (R. M. 8.)current to direct current was maintained within the range of 1:1 to 2:1,and the sheet was plated at a current density of 20 ampers per squarefoot as measured on a direct current ammeter for one minute. It was thentaken out of the bath, rinsed, and dried. The resultant sheet comprisesthe base metal stock lil containing on opposite sides thereof a layer ordeposit of sulphur (oxidizing agent) and an outer layer or deposit of areducing metal, which in this case is an alloy of nickel and cobaltcontaining approximately 98% nickel and 2% cobalt. The oxidizing layersare illustrated in Figs. 1 and 2 by the numeral Ii and the layers ofreducing metal by the numeral II. If the enamel coating is to be appliedto only one side of the base metal sheet, the other side can be shieldedduring the application of the oxidizing layer and the reducing metallayers to produce sheets of the type illustrated in Figs. 3 and 4.Whether or not this is desirable will depend to some extent upon theintended use of the final product. In most cases, the vitreous enamelcoating will be applied to both sides of the sheet.

The time required for formation of an effective oxidizing layer isshortened by using higher current densities and lengthened when lowercurrent densities are used. As previously indicated, the time of thetreatment (and, hence, the amount of the deposit formed) can bedecreased somewhat when the vitreous enamel is a higher firing enamel.The amount of the oxidizing layer required for eflectiveness inincreasing adherence may also vary, depending upon the type of reducingmetal employed, and will decrease generally the farther the reducingmetal is removed from iron (Fe++) in the electrochemical series. Theeffective amounts of oxidizing layer and reducing metal layer canreadily be determined for any particular oxidizing layer and anyparticular reducing metal layer utilizing the fundamental principlesoutlined herein.

It will be understood that the invention is not limited to any specialmethod for forming the oxidizing layer or the reducing metal layer onthe base metal stock. The reducing metal layer can be formed by thermalvaporization of the reducing metal, by electroplating, or in any othersuitable manner. Likewise, it will be understood that the method ofelectroplating given in the foregoing example is merely illustrative andrepresents a preferred way of practicing the invention. In general, theuse of alternating current is more efiective than other methods ofdepositing the oxidizing layer, and the effect is quantitative with timeand current. Instead of using alternating current, direct current can beused with a reversing switch, or direct current alone may be used.

The frequency of the current may be varied and good results may beobtained, for example, with cycle, 50 cycle and 60 cycle current in thedeposition of the oxidizing layer (e. g., sulphur) and in the productionof the abnormal wave forms preferably employed for plating the reducingmetal.

A typical vitreous enamel which can be applied to a ferrous metal basestock of the type illustrated in Fig. 2 is prepared as follows:

A frit is made from the following ingredients:

Parts by weight This material is mixed, smelted, fritted and dried inthe usual manner, and the following mill additions are made:

Frit (parts by weight)- 100 Clay -(parts by frit weight) 'l' Zirconiumsilicate opacifler do 3 Sodium nitrite do 1; Water do.... 40

This material is ground or milled to a fineness of 0-25% residue on a200 mesh screen, preferably 4-8% residue. The resultant composition isthen applied as a bisque coating to the reducing metal surface of theferrous metal stock (Fig. i or Fig. 2), prepared as described in theexample, dried and fired at a temperature below 1580 degrees Fahrenheit,e. g., in a range of 1260 degrees Fahrenheit to 1580 degrees Fahrenheit.

When the vitreous enamel composition contains a metal which is reducingwith respect to ferrous iron, e. g., antimony, arsenic, cobalt, nickelor alloys thereof, or a compound which liberates such a reducing metalunder the firing conditions, the enamel composition may be applieddirectly to an oxidised layer of the ferrous base metal of the typeillustrated in Figs. 8 and 4, which contains no coating of reducingmetal.

An important advantage of the invention lies in the fact that goodadherence is obtainedbetween vitreous enamels and ordinary or mediumcarbon steels. e. 3., those containing 0.1096 to 0.20% carbon. Steels ofthis type are widely used in making automobile bodies and for manyothuell'd ax-pussies where a very low carbon steel wo expensive or lacksthe necessary physical characteristics.

Having thus described the ,invention, what I claim and desire to secureby Letters Patent of the United States, is:

1. A method of controlling the adherence of vitreous enamels toferrousmetals which consists essentially in forming an oxidizing layercomprising essentially sulphur on a ferrous metal base stock. completelycovering said oxidising layer with a thin coating of a. nickel-cobaltalloy,

and firing a vitreous enamel on said nickel cobalt ayer.

2. A method of controlling the adherence of vitreous enamels to ferrousmetals which consists essentially in depositing a layer of sulphur on aferrous metal base stock, sealing said layer with an electroplated layerof an alloy of nickel and cobalt containing principally nickel, applyinga coating of a vitreous enamel over said alloy coating, and firing saidenamel.

3. A vitreous enamel base stock consisting essentially Of a ferrousmetal containing a thin surface layer of colloidal sulphur and ironsulphide sealed exteriorly with a thin plated coating of a predominantlynickel alloy of nickel and cobalt which is reducing in relation to ironand does not flash 03 upon thermal excitation.

4. A method of controlling the adherence of vitreous enamels to ferrousmetals which consists essentially in acid pickling said ferrous metal,depositing on the pickled surface colloidal sulphur, sealing saiddeposited sulphur with a thin coating of a plated metal from the groupconsisting of cobalt, nickel and alloys of cobalt with nickel, andfiring a vitreous enamel on said sealing coating,

5. A method of controlling the adherence of vitreous enamels to carbonsteels which consists essentially in acid pickling a steel having anaverage carbon content in excess of 0.02% but not more than 0.20%,depositing elemental sulphur as a coating on the pickled steel, sealingsaid coating of deposited sulphur with a thin coating of nickel, andfiring a vitreous enamel on said sealing coatins.

6.. A method of controlling the adherence of vitreous enamels to carbonsteels which consists essentially in acid pickling a steel having anaverage carbon content in excess of 0.02% but not more than 0.20%,depositing elemental sulphur as a coating on the pickled steel, sealingsaid coatins of deposited sulphur with a thin coating of an alloy ofnickel and cobalt containing principally nickel, and firing a vitreousenamel on said sealing coating.

7. In a method of preparing ferrous metals for vitreous enameling, thesteps which consist essentially in acid pickling a steel having anaverage carbon content in excess of 0.02% but not more than 0.20%,depositing a coating of colloidal sulphur on the pickled surface of saidsteel, and sealing said coating with a thin plated coating of a 7, metaliron the group consisting oi cobalt. nickel and alloys of cobalt withnickel.

a. In a method of preparing ferrous metals for alloy of nickel andcobalt containing principally nickel.

9. A vitreous enamel base stock consisting essentially of an acidpickled steel having an average carbon content in excess of 0.02%. andnot more than 0.20% and having thereon a coating oi colloidal sulphur,said coating being completely coated and sealed with a coating oi a thinplate of an alloy of nickel and cobalt which is predominantly nickel.

10. A vitreous enamel base stock consisting es- 2 sentially of an acidpickled steel having an average carbon content in excess of 0.02% andnot more than 0.20%, and having on the pickled surlaoe thereof a coatingof colloidal sulphur, said coating being completely coated and sealedwith a coating of a thin plate of a metal from the group consisting oicobalt, nickel and alloys of cobalt with nickel.

11. A vitreous enamel coated article obtained by firing a vitreousenamel on a steel having an average carbon content in excess of 0.02%and not more than 0.20%, which has been acid pickled and has had acoating of colloidal sulphur deposited on the pickled surface andcompletely coated and sealed with a coating of a thin plate of a metalfrom the group consisting of cobalt, nickel and alloys of cobalt withsulphur prior to firing the vitreous enamel thereon.

12. A vitreous cnamelled article obtained by firing a vitreous enamel ona steel having an average carbon content in excess of 0.02% and not morethan 0.20% which has been acid pickled and has had a coating of sulphurdeposited on the acid pickled surface and sealed with a thin plate oi analloy oi nickel and cobalt which is predominantly nickel prior to firingthe vitreous enamel thereon.

ALLAN E. CHESTER.

. REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PA'IENI'S Number Name Date 533,945 Cochran Feb. 12, 1895971,641 Rice et al. Oct. 4, 1910 1,211,218 Parker Jan. 2, 1917 1,818,579Pfleiderer Aug. 11, 1931 1,819,816 Hommel Aug. 18, 1931 1,987,967 Tanneret al Nov. 15, 1932 2,101,950 McGohan Dec. 14, 1987 2,132,438 Romig Oct.11, 1938 2,398,881 Brown et al Apr. 23, 1948 FOREIGN PATENTS NumberCountry Date 739 Great Britain Feb. 15, 1882 (of 1882) 858,886 GreatBritain Oct. 8, 1931

11. A VITREOUS ENAMEL COATED ARTICLE OBTAINED BY FIRING A VITREOUSENAMEL ON A STEEL HAVING AN AVERAGE CARBON CONTENT IN EXCESS OF 0.02%AND NOT MORE THAN 0.20%, WHICH HAS BEEN ACID PICKLED AND HAS HAD ACOATING OF COLLOIDAL SULPHUR DEPOSITED ON THE PICKLED SURFACE ANDCOMPLETELY COATED AND SEALED WITH A COATING OF A THIN PLATE OF A METALFROM THE GROUP CONSISTING OF COBALT, NICKEL AND ALLOYS OF COBALT WITHSULPHUR PRIOR TO FIRING THE VITREOUS ENAMEL THEREON.